1. TS2010 needs to be patched in order to use Multiplayer. If you look at the bottom right of the Launcher (where you select Start, Options, Content, etc) you will see a number. Please tell us this number. This is what Shane was referring to.
2. TS12 can be bought form Steam or directly from N3V Games. If you buy any version of Trainz from Steam, you won't be able to patch it and will encounter multiple compatibility problems, and you will only be able to use Multiplayer with other users with the Steam copy. If you buy from N3V Games, then you won't have any compatibility problems.
Also, TS2010 cannot join a Multiplayer session hosted by a TS12 user and vice versa.